Download Life under pressure: hydrostatic pressure in cell growth and function

Opinion
TRENDS in Plant Science
Vol.12 No.3
Life under pressure: hydrostatic
pressure in cell growth and function
Laura Zonia and Teun Munnik
Department of Plant Physiology, Swammerdam Institute for Life Sciences, University of Amsterdam, Kruislaan 318, 1098 SM
Amsterdam, Netherlands
H2O is one of the most essential molecules for cellular
life. Cell volume, osmolality and hydrostatic pressure are
tightly controlled by multiple signaling cascades and
they drive crucial cellular functions ranging from exocytosis and growth to apoptosis. Ion fluxes and cell shape
restructuring induce asymmetries in osmotic potential
across the plasma membrane and lead to localized
hydrodynamic flow. Cells have evolved fascinating strategies to harness the potential of hydrodynamic flow to
perform crucial functions. Plants exploit hydrodynamics
to drive processes including gas exchange, leaf positioning, nutrient acquisition and growth. This paradigm is
extended by recent work that reveals an important role
for hydrodynamics in pollen tube growth.
The pivotal role of water in the origin, function and
proliferation of cellular life
Life is all about aqueous chemistry and reactions that
occur at surfaces and interfaces. The unique physical
properties of water not only promoted the emergence of
cellular life but also set limits on effective cell dimensions
within which viability and reproduction can be maintained
[1]. It was crucial for cell function that osmolality, membrane tension and hydrostatic pressure (see Glossary)
were tightly controlled. Thus, early in evolution, cells were
confronted with the problem of how to control their volume
and regulate the flow of water across the membrane. It is
believed that solutions to this problem arose with the
earliest protocells, and these formed the basis of volume
regulation during the development of cellular complexity
[1]. The list of processes that are controlled by cell volume
and hydrodynamics reads like the book of life itself – it
includes growth and proliferation, membrane transport,
exocytosis, endocytosis, cell shape changes, hormone signaling, metabolism, excitability, neural communication,
cell migration, nutrient delivery, waste filtration, necrosis
and apoptosis [2,3].
For cells enclosed by a cell wall, growth is dependent on
deposition of new wall materials at the cell surface coupled
with osmotic pressure to drive expansion of the cell. There
has been some debate about whether plant cell growth is
primarily controlled by modulations of cell wall stiffness or
by changes in osmotic pressure (Box 1). Deposition of new
wall materials is not in itself sufficient to promote growth
[4]. However, recent work is revealing a crucial role for
Corresponding author: Zonia, L. ([email protected]).
Available online 12 February 2007.
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osmotic pressure, cell volume and hydraulic conductivity in
driving cell shape restructuring and growth [5–14]. In this
Opinion article, we will present the case for hydrodynamic
control of plant cell growth.
To build this case we will briefly discuss some important
aspects of cell hydrodynamics that support this alternative
model: the initial events in cell volume perception, mechanisms that initiate cell volume recovery, and several
functions regulated or driven by cell volume that are
important for growth. We will discuss evidence from
animal systems where much work has been done, but will
focus on the similarities discovered to date in plant cells.
The last section will discuss recent work on the role of
volume and pressure in pollen tube tip growth. It is hoped
that the concepts presented in this Opinion article will
stimulate new ways of thinking about plant cell growth and
function.
Sensors and controllers in cell volume regulatory
networks
Mechanosensitive ion channels
Water surrounds cells, penetrates into the plasma
membrane and cytosol, and affects the local geometry of
the lipid bilayer and membrane proteins [15]. Ions or osmolytes perturb the aqueous network and affect membrane
tension and osmotic potential. In bacteria, milli-osmolar
changes in water concentration are sufficient to shift the
osmotic pressure across the plasma membrane and generate
stretch and compression forces along the plane of the lipid
bilayer [16]. These forces gate mechanosensitive (MS) ion
Glossary
Anisotropic: exhibiting unequal properties along different axes.
Electro-osmosis: the movement of a polar fluid through a selectively permeable membrane or porous material, or along a charged surface, under the
influence of an electric field.
Hydraulic conductivity: the movement of fluid through pores or confined
spaces under pressure, depending on the intrinsic permeability of the material
and on the degree of saturation.
Hydrodynamics: the dynamics of fluids in motion; the forces exerted by fluids
in motion.
Hydrostatic pressure: the pressure exerted by a static fluid, depends on the
fluid depth, density and gravity; independent of shape, total mass or surface
area of the fluid.
Osmotic potential: the potential for water to move across a selectively
permeable membrane.
Osmotic pressure: the force exerted against a selectively permeable membrane
by a solution that has different solute concentrations on either side of the
membrane.
Viscoelastic: a material that exhibits both viscous and elastic properties, and
shows time-dependent strain when a stress is applied.
1360-1385/$ – see front matter ß 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.tplants.2007.01.006
Opinion
TRENDS in Plant Science
Box 1. What drives plant cell growth – cell wall loosening or
osmotic pressure?
There is agreement that both cell wall properties and turgor
pressure have roles in growth, but there is disagreement about
what drives the initial event of cell enlargement. Current theory
considers that cell wall loosening occurs first, which then allows
expansion of the cell wall because of the decreased rigidity. As
expansion starts there is a simultaneous decrease in intracellular
pressure, which then promotes passive entry of H2O down its
osmotic potential gradient, causing further expansion of the cell
[32,77,78]. There is considerable evidence to support this theory.
Expansin proteins promote cell wall stress relaxation, and their
expression and activity correlate with growth [79]. Experimental
techniques that increase cell wall rigidity in pollen tubes also
decrease growth rates [32,59,80]. However, a recent cytomechanical
study revealed that there is no significant change in the viscoelastic
properties of the pollen tube cell wall in the apical growth zone
during repeated deformations [56]. This suggests that cycles of cell
wall loosening cannot be the driving force for oscillatory growth.
Furthermore, recent analyses succeeded in resolving previously
undetected data and show that increased pressure induces expansion of the cell wall and drives growth in Chara cells [5,6]. The
mechanics of cell wall expansion in the apical dome is similar for
root hairs, Chara and Nitella rhizoids, and fungal sporangia in spite
of differences in cell wall composition, indicating that it is an
emergent feature of tip growth [81]. Thus, there is evidence that
pressure induces growth in plant cells. A new model called loss of
stability (LOS) has been proposed [7,8]. The LOS model considers
that turgor pressure increases gradually to a critical point that is
determined by the cell wall geometry and material properties, at
which point the pressure causes a loss of stability of the cell wall
that manifests as wall extension and growth. A recent examination
of pollen tube growth extends this work and reveals a role for
hydrodynamic flow. This work shows that a driving force for growth
in pollen tubes is mediated by cyclical volume increases in the
apical region [9]. Cell volume increases are known to promote
exocytosis (see text), and turgor pressure can drive polysaccharide
insertion into cell walls and promote cell wall assembly [10,11]. In
summary, there is increasing evidence that osmotic pressure and
hydrodynamics have crucial roles in driving plant cell growth.
channels (MscS, MscL, TRP) that function as osmotic safety
valves and initiate processes leading to cell volume recovery
[16,17]. MS channels arose early in evolution and are present across all three domains of Archaea, Bacteria and
Eukarya. Indeed, pressure-induced activation of MS channels is postulated to be one of the first signal transduction
cascades that arose with the onset of cellular life [16,17]. In
plants, MS ion channels have been identified that are
involved in cell volume perception and regulation. Arabidopsis mesophyll cells have a MS anion channel that is
specifically gated by convex curvature of the plasma membrane, such as occurs during cell swelling [18,19]. Tobacco
suspension cells contain a MS anion channel that mediates
fluxes great enough to induce cell volume changes [19].
Furthermore, an MscS-like channel was shown to control
plastid size and shape in Arabidopsis [20]. Interestingly, the
bacterial MscS channel is pressure-gated, voltage-dependent, and selective for anions [17]. A MS Ca2+ channel has
been identified in lily pollen tubes that is thought to mediate
Ca2+ influx at the apex [21].
Aquaporins
Plasma membrane permeability to water is greatly
enhanced by aquaporin channels that allow the flow of
H2O (and some small neutral solutes) across membranes
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91
[22]. It has been proposed that aquaporins might function
as osmosensors and regulators of cell volume in animal,
plant, fungal and bacterial cells [23]. Plants express more
aquaporin homologs than are expressed by animals,
reflecting the importance of regulating water flow for plant
cells. Plant aquaporins have high levels of expression in
cells undergoing large volume changes or high water flux
rates, including pollen coat and stomatal guard cells
[24,25].
Activation of ion fluxes during cell volume perturbation
and recovery
There is a dynamic balance between the osmotic potential
across the plasma membrane and cell volume, which is
dependent on intracellular metabolic flux and extracellular conditions. In neuronal cells undergoing intense activation, high ionic flux rates cause localized changes in the
osmotic balance across the plasma membrane that induces
water flow out of the cells and invagination of the plasma
membrane [26]. During homeostatic regulation in many
cell types, any perturbations to the osmotic balance rapidly
activate mechanisms involved in cell volume recovery. The
first responses involve changes in ion flux across the
plasma membrane, which can be detected in animal, plant
and fungal cells within 1–10 min during volume recovery
(Figure 1) [2,27–29]. Changes in ionic flux rates rapidly
result in H2O flow across the plasma membrane and induce
changes in cell volume, and vice versa. Hyperosmotic shifts
or cell shrinkage induce K+, Cl and Na+ uptake, and H+
efflux, which cause H2O to flow into the cells and leads to
regulatory volume increase. Conversely, hypo-osmotic
shifts or cell swelling induce K+ and Cl efflux and H+
influx, which cause H2O to flow out of the cells and leads to
regulatory volume decrease. Cell swelling and increased
hydrostatic pressure induce influx of extracellular Ca2+ in
animal cells [2,30], and there is evidence that swelling
induces Ca2+ influx in pollen tubes [31]. Ca2+ influx at the
apex is an important regulator of pollen tube growth [32].
Activation of signaling cascades during cell volume
perturbation and recovery
Secondary responses to cell volume changes include
activation of downstream signaling pathways that target
a plethora of cell functions (Figure 1) [2,33–36]. Numerous
studies have shown that the levels of signaling lipids are
increased by osmotic stress. Hypo-osmosis and cell swelling
induce increases in phosphatidic acid in pollen tubes, unicellular green algae and erythrocytes. Hyperosmosis and
cell shrinkage induce increases in phosphatidylinositol-bisphosphates in animal and plant cells. Osmotic stress also
activates MAPK cascades and induces gene expression. Cell
volume changes impact cytoskeletal organization, and Rho
GTPases are a major point of convergence to integrate
membrane signals and cytoskeletal organization. In animal
cells, there is increasing evidence that Rho GTPases are
regulated by cell volume and intracellular ionic strength
[2,37,38]. Rac is activated by both cell swelling and cell
shrinking, and specificity appears to be linked to subcellular
localization. Rho is activated by cell shrinking, and its
presence is required for signaling cascades that are activated by cell swelling. The mechanisms involved in Rho
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Figure 1. Cell volume regulatory networks in (a) animal and (b) plant cells. First responses to hypertonic shrinking and hypotonic swelling are mediated by changes in ion
fluxes across the membrane, which drive the influx or efflux of H2O and lead to cell volume recovery. Hypertonic shrinking in plant cells causes the protoplast plasma
membrane to retract from the cell wall (plasmolysis), remaining attached by thin cytoplasmic projections called Hechtian strands. Secondary responses to cell volume
changes are mediated by phospholipid signaling cascades, protein kinase cascades and Rho GTPases. Question marks denote known functions in animal cells that might be
possible but have yet to be tested in plant cells. Abbreviations: AA, arachadonic acid; cAMP, cyclic AMP; DAG, diacylglycerol; DGPP, diacylglycerol pyrophosphate; ERK1/2,
extracellular signal-regulated kinases; mTOR, mammalian target of rapamycin; N, nucleus; PI3K, phosphoinositide 3-kinase; PtdIns(3,4)P2, phosphatidylinositol (3,4)bisphosphate; PtdIns(3,5)P2, phosphatidylinositol (3,5)-bisphosphate; PtdIns(4,5)P2, phosphatidylinositol (4,5)-bisphosphate; PtdOH, phosphatidic acid; SIMK, stressinduced MAPK; V, vacuole.
GTPase activation in response to cell volume changes are
currently under investigation. In plant cells, recent work
indicates the importance of Rho/Rac GTPases for growth
and development [39]. Future research should investigate a
potential link between Rho/Rac GTPases and volume regulation in plant cells.
Hydrodynamic flow drives crucial cellular functions
Cell volume crucially affects membrane tension and
curvature, molecular crowding in the cytosol, and intracellular ionic strength [40–42]. Because of this, processes
ranging from growth and proliferation to necrosis and
apoptosis are controlled by cell volume in Bacteria and
Eukarya [2,3]. Furthermore, many cell types have evolved
fascinating strategies that enable them to harness the
potential of osmotic gradients and hydrodynamic flow to
drive crucial cellular functions. Some functions occur more
readily under certain cell volume conditions, such as exocytosis, endocytosis, neural activation and enzymatic reactions. Some functions in plants and fungi can only be
driven by exploiting hydrodynamic flow, such as stomatal
guard cell opening and closing, leaf pulvini motor organ,
mechanical traps of carnivorous plants, and fungal appressorial penetration. A survey of cellular functions that are
driven by or dependent on cell volume is presented in
Table 1. In this Opinion article, we will briefly discuss a
well-characterized plant cell system that is driven by
hydrodynamics – stomatal aperture and membrane
dynamics in guard cells.
Guard cells undergo osmotically induced shrinking and
swelling and thereby control stomatal aperture size, which
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is crucial for the control of transpiration and gas exchange.
This reversible hydrodynamic loading–unloading is largely
driven by the accumulation or loss of K+ and Cl [43].
During reversible cell swelling, the intracellular osmolarity can change by as much as 50% [44]. Surface area varies
linearly with cell volume changes and can increase by 40–
50% [45]. Furthermore, the swelling–shrinking cycles of
guard cells can oscillate rapidly, enabling tight control of
stomatal aperture size [46]. In summary, these results
indicate that ion flux changes rapidly modulate cell
volume.
There is a strong correlation between membrane
tension and vesicle insertion or retrieval in many different
cell types. Increases in cell volume or membrane tension
stimulate exocytosis, whereas decreases in cell volume or
membrane tension stimulate endocytosis [47]. The situation is similar in guard cells. Cell volume increases drive
exocytosis and surface area expansion, whereas cell
volume decreases drive endocytosis and surface area contraction [44,45]. There is also evidence for constitutive
membrane turnover in guard cells [44]. Continuous membrane turnover might enable plant cells to respond rapidly
to small shifts in the osmotic potential across the membrane and, thus, control membrane tension.
Vectorial hydrodynamic flow and hydrostatic pressure
surges can trigger localized cell expansion and growth
There is increasing evidence that cell shape restructuring
and polarized growth are initiated by transiently nonequilibrated hydrostatic pressure surges pushing against
the cell boundary [9,12–14]. Recent work in animal and
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Table 1. A survey of cellular functions driven by or dependent on cell volume and hydrodynamics
Organism
Vertebrate
Human
Human
Human
Human
Human
Human, rat, mouse, Xenopus
Human, pig, rat
Mouse
Mouse
Chick embryo
Function
Refs
Exocytosis and endocytosis
PLD-mediated exocytosis and endocytosis
Neural activation in visual cortex
Kidney filtration of plasma
Liver metabolism
Proliferation and apoptosis
Rho/Rac GTPase activation and reorganization of actin cytoskeleton
Melanoma blebbing
Neurotransmitter release at neuromuscular junction
Neural axon growth
[47]
[62]
[63]
[64]
[65]
[66,67]
[37,38]
[12]
[68]
[50]
Plant
Arabidopsis, Vicia, Maize
Nicotiana, Chara
Chara
Vicia, Arabidopsis, Nicotiana
Samanea saman
Nicotiana tabacum
Nicotiana tabacum
Venus flytrap
Exocytosis and endocytosis in guard cells
Cell growth
Polysaccharide insertion into cell walls; cell wall deposition and assembly
Stomatal aperture, stomatal oscillations
Leaf positioning
Leaf unfolding
Anther dehiscence
Trap closure
[44,45]
[5,6,9]
[10,11]
[43,46]
[69]
[70]
[25]
[71]
Slime mold
Dictyostelium
Blebbing
[13]
Fungi
Neurospora crassa
Magnaporthe grisea
Ascobolus immersus
Hyphal mass flow
Appressorial penetration of host tissues
Pressurized spore dissemination
[51]
[72,73]
[74]
Bacteria
E. coli
E. coli
Growth
Chemotaxis
[75]
[76]
bacterial cells indicates that the cytoplasm is highly
non-uniform and structured as a porous contractile elastic
network composed of cytoskeletal filaments and organelles, infiltrated with an electrolytic interstitial fluid,
similar to a fluid-filled sponge [12,42]. It is proposed that
the anisotropic nature of these structural networks,
electrolyte pathways and pools ultimately results in vectorial electrochemical gradients within the cell [42]. The
result of this is that hydrostatic pressure cannot instantaneously propagate throughout the cytoplasm and is
highly dependent on the hydraulic and electrochemical
conductivity of the network [12,42]. Thus, hydrostatic
pressure can be transiently non-equilibrated within the
cell. In regions with weaker cortical cytoskeletal arrays, a
hydrostatic pressure surge can push the plasma membrane
outward to create a bulge [12,13]. In pollen tubes and root
hairs, there is evidence that the apex has weaker cortical
arrays than distal regions because plasmolysis induces
retraction of the protoplast from the apex (Figure 2a)
[48,49]. Bulge formation has been modeled as the initial
event for polarized growth of root hairs, trichomes and
pavement cells [14].
Several cell types grow by an anisotropic process called
tip growth, including neurons, pollen tubes, root hairs and
fungal hyphae. There have been relatively few studies of
the role of osmotic pressure and cell volume in tip-growing
cells, but the most recent data indicate a role for hydrodynamics. Growth rates of neuronal axons were increased
by as much as sixfold by hypo-osmotic shifts in the extracellular medium, whereas hyperosmotic shifts reduced
growth rates [50]. Osmotic gradients have a role in driving
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mass cytoplasmic flow and hyphal extension in Neurospora
[51]. As noted above, hydrostatic pressure surges might
drive bulge formation in root hairs and pollen tubes [9,14].
Below we consider pollen tube growth.
Properties of cytoplasm and cell wall
The pollen tube apical region differs from distal regions in
several key parameters. Actin arrays are present as a fine
cortical fringe in the apical dome and as longitudinal
filaments in the distal tube [52]. The distribution of
vesicles and organelles differ in that the apical dome is
enriched with vesicles whereas organelles are largely
excluded [53]. Oscillations in ion fluxes that are correlated
with growth rate oscillations are prevalent at the apex and
along the tube flanks in the apical region [32,54,55]. This
might confer transient and localized electro-osmotic
activity during the growth cycle. The apical region behaves
viscoelastically whereas the distal region behaves elastically and with greater stiffness [56]. The tips of fungal
hyphae also have greater viscoelasticity and hydrophilicity
than distal regions [57]. Finally, it is the apical 50 mm
region that undergoes volume changes in response to
osmotic or ionic perturbations (Figure 2a) [33,58]. Taken
together, these results indicate that the cytoplasm of pollen tubes is highly non-uniform and that apical and distal
domains are likely to have different hydraulic conductivities.
Manipulation of the osmotic potential difference across
the plasma membrane rapidly modulates apical volume,
cell wall expansion, and pollen tube growth rates (see
also Figure 2a) [9,33,58]. Increasing the culture medium
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Figure 2. Hydrodynamics in pollen tube growth. (a) The pollen tube growth rate oscillator depends on apical volume and pressure. In isotonic conditions, the chart shows
that the average growth cycle period is 50 s. Upon shifts to hypertonic conditions, apical shrinking occurs and cells can undergo plasmolysis at the apex; the chart shows
that average growth cycle period is 100 s. Upon shifts to hypotonic conditions, apical swelling occurs; the chart shows that average growth cycle period is 25 s. (b) D2O
affects pollen tube morphology and growth. Pollen tubes were germinated for 2 h in normal medium and then grown for 12 h in normal medium (Control) or in D2O-based
culture medium (D2O). D2O flows through cells with different kinetics than those of H2O, reducing growth rates and inducing cell wall thickening, branching, and irregular
pollen tube diameters. (c) Proposed model for apical volume oscillations in pollen tubes. The chart shows an example of two cycles of pollen tube growth. The growth rate
decreases during phase I, whereas phase II has maximally increasing growth rate. During phase I, electro-osmosis and hydraulic conductivity drive H2O into the pollen tube
and lead to gradual volume increase in the apical region. Apical swelling and increased pressure at the apical plasma membrane induce stress relaxation of the cell wall and
trigger the start of the growth cycle. During phase II, vectorial mass flow and regulatory volume decrease mechanisms result in H2O efflux at the apex, which drives cell
elongation and growth. Scale bars = 10 mm. Portions of (a) and (b) reproduced, with permission, from Ref. [9].
osmolality and stiffness induce decreases in pollen tube
growth and decrease the abundance of cell wall pectins
[59]. These results suggest that the mechanical properties
of the cell wall might be adjusted to balance rather than
control the dynamic interaction between extracellular and
intracellular pressures. This would enable fine-tuning of
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cell wall mechanical properties in different osmotic
environments and during different phases of cellular activation. Expansins and other wall-modifying enzymes
would have particularly important roles in rapidly growing
cells to maintain optimum cell wall extensibility with
respect to this dynamic balance.
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Cell volume, hydrodynamics, and hydrostatic pressure
surges
Pollen tube cell volume has a crucial role in fertilization, is
exquisitely sensitive to the osmotic potential difference
across the plasma membrane and is correlated with growth
[9,33,58,60]. When a pollen tube reaches the ovule it stops
growing and undergoes a massive increase in volume and
pressure that causes the cell to explode at the apex,
propelling the sperm cells into the ovule sac [60]. Apical
swelling is a rapid and generalized result of growth inhibition, suggesting that during normal growth there is a
mechanism that dissipates or expends this pressure
buildup.
A recent investigation of apical volume in growing
Nicotiana tabacum pollen tubes revealed an important
role for hydrodynamics (Figure 2) [9]. The apical region
undergoes volume oscillations that have the same frequency as growth rate oscillations but are phase-shifted
by 1808, so that the start of the growth cycle occurs as
volume reaches maximum, and growth rate reaches maximum as volume reaches minimum (Figure 2a, Isotonic).
Experimental manipulation of apical volume is sufficient
to reset the growth rate oscillator [9]. Hypertonic treatment decreases pressure and causes the average growth
period to double, whereas hypotonic treatment increases
pressure and causes the average growth period to halve
(Figure 2a). Osmotic manipulation of volume is sufficient to
reset the growth rate oscillation frequency (period) – not
just the amplitude – which indicates that hydrodynamics
has a crucial function in driving the pollen tube growth rate
oscillator. When extracellular H2O in the culture medium
is replaced with 2H2O (deuterium oxide, D2O), pollen tube
growth and cell morphology are adversely affected
(Figure 2b). Transfer back into H2O medium reverses
the effects on growth and morphology [9]. Vectorial mass
flow of cytoplasm toward the apex has been observed in
fungal hyphae and pollen tubes [51,58]. There is evidence
from Raman microscopy that polarized growth is maintained at least in part by the flow of H2O out of the pollen
tube apex [9].
function as a self-organizing, cyclic and dissipative
mechanism that sets the frequency of growth rate oscillations. When cycles of regulatory volume decrease–
increase are uncoupled, growth rate would be stochastic.
When the cycles are coupled, hydrostatic pressure surges
would provide a motive force that enables the pollen tube to
penetrate the stigma and grow through the style like a
hydraulic drill.
Hydrodynamics and pollen tube growth
These results lead to a new model for pollen tube growth
(Figure 2c). This model proposes that growth rate oscillations are driven by cycles of vectorial hydrodynamic flow
that cause volume increases and decreases in the apical
region. Hydrodynamic flow is likely to result from electroosmotic activity and hydraulic conductivity in the apical
region [9,32,33,52–55,58]. Volume oscillations would spontaneously emerge from activation of cell volume recovery
mechanisms (Figure 1) that cycle around an attractor for
optimal intracellular osmolality. Volume increases result
in gradually increasing hydrostatic pressure surges that
promote exocytosis, activate mechanosensitive channels
and trigger cell wall stress relaxation and the start of
the growth cycle. Apical swelling simultaneously activates
mechanisms involved in regulatory volume decrease
(Figure 1b, hypotonic swelling). Vectorial mass flow and
hydrodynamic efflux at the apex drive polarized cell
elongation while also promoting compensatory hydrodynamic influx along distal regions. These dynamics would
Acknowledgements
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Conclusions and perspectives
Hydrodynamics and hydrostatic pressure are among the
most fundamental physical properties that determine cell
form and function. Cells rapidly respond to changes in
volume and osmotic potential differences across the
plasma membrane. Hydrostatic pressure changes can generate stretch or compression forces along the plasma membrane and activate mechanosensitive ion channels. This
simple switch is postulated to be one of the oldest sensory
transduction processes that evolved with the onset of
cellular life. There is increasing evidence that hydrodynamics and/or hydrostatic pressure have important roles in
cell shape and structure, exocytosis and growth, in animal
and plant cells, algae, slime molds, oomycetes and fungal
hyphae [5,6,9–14,44,45,51].
Evidence is emerging that the cytosol consists of a
highly anisotropic and elastic network composed of cytoskeletal arrays and organelles, infiltrated with interstitial
electrolytic pathways and electrochemical gradients
[12,42]. Because of this, localized hydrostatic pressure is
strongly dependent on the local hydraulic conductivity of
the network. All the available evidence indicates that
apical and distal regions of pollen tubes differ in their
hydraulic conductivity. Osmotic potential gradients and/
or electro-osmosis are likely factors that induce apical
volume oscillations and hydrostatic pressure surges in
pollen tubes. It has been proposed that oscillations in
membrane ion transport could occur in any plant cell or
tissue under suitable conditions [61]. It will be interesting
to discover if hydrostatic pressure surges are a general
mechanism of plant cell growth.
We apologize to scientists whose work could not be cited due to space
limitations. We thank Michel Haring for critical reading of the
manuscript. Research in T.M.’s laboratory is currently supported by the
Netherlands Organization for Scientific Research (NWO 813.06.003,
863.04.004 and 864.05.001).
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Plant Science Conferences in 2007
CSHL Meeting: Plant Genomes
15–18 March 2007
Cold Spring Harbor Laboratory, New York, USA
http://meetings.cshl.edu/meetings/plants07.shtml
Model Legumes Congress (MLC2007)
24–28 March 2007
Tunis (Tunisia)
http://www.ecopark.rnrt.tn/mlc2007/
SEB Main Meeting 2007
31 March – 4 April 2007
Glasgow, UK
http://www.sebiology.org/Meetings/pageview.asp?S=2&mid=91
Royal Society Discussion Meeting - Revealing how Nature uses Sunlight to Split Water
23–24 April 2007
London, UK
http://www.royalsoc.ac.uk/
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